Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of the common general knowledge in the field.
Sterilisers are used in the medical, food and packaging industries to kill and thereby prevent the transmission of transmissible agents such as spores, fungi, and bacteria. A typical steriliser creates a set of physical conditions in a sterilisation chamber that effectively kills nearly all of these transmissible agents.
Contacting articles in need of sterilisation with sterilant aerosols is one known method of sterilisation. Referring to FIG. 1, a typical aerosol sterilisation apparatus SA has a sterilisation chamber SC with inlet and an outlet valves (IV and OV, respectively), an aerosol generator 2 (typically a nebuliser) in fluid communication with the chamber via the inlet valve and a fan F upstream of, and in fluid communication with, the aerosol generator.
In use, an article requiring sterilisation is placed in the chamber, which is then sealed. The aerosol inlet valve is opened and the outlet valve is closed. The fan is engaged, which creates a gas stream through or the past the aerosol generator, into the chamber. A passive vent in the sterilisation chamber allows for pressure equalization as required, to permit gas flow in and out of the sterilisation chamber. The aerosol generator, which contains the desired sterilant, is then activated, putting a large number of small sterilant droplets into the gas stream. The droplets are carried by the gas stream to create an aerosol which travels into the sterilisation chamber. The sterilant then acts upon the contents of the chamber, killing pathogenic organisms as required.
One type of nebuliser which has proved to be well suited for sterilization applications is an ultrasonic nebuliser.
In an ultrasonic nebuliser, a sterilization liquid is placed in a cup which sits above and in contact with an ultrasonic transducer. Typically, the ultrasonic transducer is a piezoelectric crystal that changes size or shape in response to electrical stimulus. The application of alternating current to the crystal at high frequencies (of the order of several MHz) leads to the crystals vibrating at a corresponding frequency. This energy is in turn transferred to the sterilization liquid. The cup and transducer may typically be configured to focus the distribution of energy in the sterilization liquid. The energy causes small microdroplets to be formed from the liquid and become airborne. Typically, there are many such particles which together form a nebulant or mist of aerosol particles. This nebulant is then delivered to the sterilizing chamber.
One problem with such an arrangement is that if the sterilizing liquid is fully consumed and the cup becomes empty, then any continuing operation of the device is likely to result in damage—the energy is not dissipated by the nebulant, but rather is retained by the transducer and cup, causing the arrangement to overheat which can irreversibly damage the transducer unit, for example, by delamination or depolarisation of the transducer.
Another problem of such an arrangement is that if the sterilizing liquid overfills the cup, the transducer experiences an increase in load and the system efficiency can reduce, potentially causing an undersupply of sterilizing aerosol and a resulting failure to sterilize.
“Sterilization” is generally defined as a process capable of achieving a log 6 reduction in concentration of spores. “Disinfection” is a similar process, the difference being that it results in a lesser degree of biocidal effect. “Sterilization” includes “disinfection” and “disinfection/sterilization” is an abbreviation for “disinfection and/or sterilization”. In the present application, “disinfection” and “sterilization” are used interchangeably.
Because of the constructional requirements of ultrasonic sterilizers, it is not a trivial task to in determine when there is an appropriate level of liquid in the ultrasonic cup. The sterilizing liquid usually contains hydrogen peroxide, which is highly toxic and corrosive, requiring the cup and the transducer to be maintained in a tightly sealed system. Any sensing means needs to be sufficiently robust to withstand repeated exposure to toxic and corrosive peroxide liquid and vapour, possibly at high temperatures. In addition, because of the fluid in the environment and the possibility of splashing, the sensor need to be highly accurate in its ability to avoid false positive and false negative results, which could lead to either destruction of the transducer element or an undersupply of sterilizing aerosol.
Thus, there is a need for sensor for determining the level of liquid in an ultrasonic cup which is robust enough to endure repeated exposure to sterilization conditions.